Controlled release compositions are being recognized as the technology of the future to provide continuing activity over an extended period of time without the need for additional applications of the active agent. Controlled release compositions are useful with animal repellants, pesticides, herbicides, fungicides, plant growth stimulants, fertilizers, and drugs. Controlled release compositions allow application of a lesser amount of active agent to achieve better control than application of the active agent directly (which generally results in loss through leaching or otherwise before the active agent can be effectively used). Four mechanisms are commonly employed to obtain controlled release:
(1) desorption from strong sorbents, like silica gel, mica, and activated charcoal; PA1 (2) diffusion; PA1 (3) erosion of biodegradable barrier materials; and PA1 (4) release after retrograde chemical reactions, such as hydrolysis, thermodynamic dissociation, or microbial degradation.
The delivery rate of a chemical from a controlled release system is primarily influenced by the architecture of the system, the properties of the impregnant and of the rate-controlling matrix, and the driving force liberating the impregnant from the matrix. Physical controlled release compositions are either reservoir systems or monolithic systems. In a reservoir system, the active agent is encapsulated within a rate-controlling membrane. The membrane permeability and the membrane configuration determine the release rate. In a monolithic system, the active agent is dissolved or dispersed throughout a matrix, such as a polymer.
One commercial reservoir system uses a hollow fiber to hold the active agent, such as an insect pheromone. The release of the active agent from the fiber is diffusion controlled. This system is beneficial for volatile liquids, yet it is expensive because of the cost of manufacture of the tubes. Many other controlled-release compositions are known, especially for insecticides, drugs, and fertilizers. Most are encapsulation reservoir systems similar to the hollow fibers but depending on diffusion through a semipermeable membrane.
When cellulose is swollen in water and the water is replaced by a solvent through a series of solvent exchanges, the final solvent is often trapped inside the cellulose structure upon its drying. The entrapped solvent is released by contacting the cellulose with water. Kistler, 35 J. Phys. Chem. 52 (1932).
Because investigators have been interested primarily in enhancing the rates of chemical reactions by making inclusion cellulose, no systematic study of the inclusion process has been conducted. Believing that the solvents were entrapped in the amorphous or intercrystalline regions of the cellulose stucture, most investigators thought that the molecular size of the solvents used must be small (less than about 10 angstroms). Blackwell, Kolpak, and Gardner, Cellulose Chemistry and Technology, 48 ACS Symp. Ser. 42 (1977). The release mechanism was thought to include destruction of the crystalline region of the cellulose during swelling. Small amounts of chemical solvents, such as ethylene glycol, methanol, ethanol, acetone, toluene, benzene, carbon tetrachloride, pyridine, n-hexane, chloroform, cyclohexane, isopropanol, n-butanol, bromobenzene, and dichloroethane, were released using water, ammonia, or sodium hydroxide as a swelling agent.